South Korean soldiers walk near the demilitarised zone separating the two Koreas on May 27, 2009. North Korea on Wednesday threatened a military strike against the South.

Kim Kyung-Hoon/Reuters

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Washington and Boston — Where does North Korea's nuclear program go from here? That's a key technical question in the wake of Pyongyang's underground test of a nuclear device on May 25.

Proliferation experts will want to determine, in particular: the nature of that nuclear device, whether it went off as expected, and whether the North Koreans now intend to produce more fissile material for a potential weapons stockpile.

"There's a tremendous amount we do not know about North Korea," said Richard Bush III, a former US National Intelligence Officer for East Asia, at a Brookings Institution seminar on the nuclear crisis May 27.

Scientists at the Lamon-Doherty Earth Observatory in Palisades, N.Y., place the yield of the May 25 blast at between 2.2 and 4 kilotons of TNT – significantly larger than North Korea's 2006 experiment.

The 2006 test still "would have been a disaster if it had gone off in a metropolitan area," says Won-Young Kim, a seismologist at the observatory who specializes in detecting underground nuclear tests. "[But] this one was bigger."

Mr. Kim and his colleagues base their estimate on seismograph readings from several sites in the Pacific and central Asian regions, as well as one in China that was only about 230 miles from the blast zone. The China site is run by a group of international universities and the US Geological Survey.

They are reasonably certain the shaking of those seismographs was caused by fissile material reaching critical mass. The blast carried the unique signature of an underground explosion, rather than that of an earthquake.

It is possible to fake a nuclear blast with conventional explosives, but the Lamon-Doherty team considers that explanation to be implausible in this case. There is some uncertainty to their yield estimate because no one outside of North Korea knows exactly how deep the device was planted.

The question of whether the explosion was successful, however, is more difficult to answer. Clearly, it was not a flop. But no one outside of a tight coterie in North Korea has any idea what kind of explosive yield their device was designed to produce.

If it was supposed to produce about a 4 kiloton explosion, that is one thing. But it could have been intended to produce 20 or 30 kilotons. The Nagasaki bomb of World War II produced an explosion equal to about 21 kilotons of TNT, for instance.

"The missing piece of information is what they were trying to achieve," says Paul Brannan, a senior research analyst at the Institute for Science and International Security.

Many experts believe that North Korea's nuclear design is an implosion type, as was the Nagasaki weapon. Implosion bombs squeeze a round core of plutonium to produce their explosive power. North Korea is known to have plutonium – an estimated six to eight bombs' worth, produced from spent nuclear reactor fuel.

By contrast, gun-type nuclear bombs need uranium. They create a critical mass by firing one cylinder of highly enriched uranium into another. They are much easier to engineer, but Pyongyang has long denied that it has any highly enriched uranium, despite a few hints to the contrary puzzled out by US intelligence.

The yield produced by the recent test was within the range of scientific expectation for a first-generation implosion bomb, according to Charles Ferguson, senior fellow for science and technology at the Council on Foreign Relations.

"So I believe that North Korea is probably happy with this result ... this is a bomb that can wreak massive destruction," says Mr. Ferguson.

It's still possible that the device did not produce an explosion as big as intended, of course. And a nuclear "device," cobbled together for an underground test, is in any case a less-precise item than an actual warhead, engineered to be small enough to fit on top of a missile.

That is a level of accomplishment that Pyongyang has yet to achieve, Ferguson says.

"There is still considerable doubt that North Korea can field such a weapon on a long-range ballistic missile," he adds.

Of that, about 28 to 50 kilograms (62 to 110 pounds) were thought to be in a separated form and usable for weapons.

This amount of fissile material would produce one 5 kilogram (11 pound) test device, plus a hypothetical arsenal of three missile warheads, and three heavier bombs.

But is North Korea now gearing up to separate more plutonium? That's a question raised by reports in the South Korean press, which suggest that North Korea may have resumed reprocessing spent nuclear fuel at its plutonium separation plant at Yongbyon.

These reports cite US intelligence imagery showing steam rising from the facility.

In one sense, it would not be surprising for North Korea to fire up the plant again, given that Pyongyang vowed in April it would do just that.

But commercial imagery obtained by ISIS in late May does not show any smoke rising from the chimney of the coal-fired stem plant at Yongbyon, nor any plume rising from the stacks at the reprocessing plant.

"Steam or no steam, we doubt they are actually doing plutonium separation at the reprocessing plant now," says Paul Brannan of ISIS. "It is too soon."